30
June 2012 APC
Cost-effective solutions to reduce
mercury air and wastewater emissions
Richard Mimna, Rebecca L. Stiles, Jianwei Yuan, Bruce A. Keiser, John Meier
With the issuance of the Mercury Air Toxics Standards (MATS) in December 2011, the U.S. EPA has
passed lasting federal guidelines for mercury air
emissions from coal-fired power plants. To reach
the 1.2 lb Hg/TBtu air emission limit specified in
the MATS regulation, many power plants will require additional products and technologies to
boost the mercury capture co-benefits of their existing air quality control devices. Nalco has developed
a suite of technologies practiced by the industry to
enhance mercury oxidation, prevent mercury reemission from scrubbers, and bring scrubber
wastewaters into compliance with effluent limits.
These technologies, as well as two case studies that
document their use and effectiveness, are discussed
in this article.
A
fter years of studies on mercury emissions
control, legal setbacks, and several rewrites,
the U.S. EPA appears to have finally succeeded in passing lasting federal guidelines for mercury air emissions from coal-fired power plants, with
the issuance of the Mercury Air Toxics Standards
(MATS) in December 2011 (1). In the midst of all the
legal wrangling and debate, the agency always maintained that much of the mercury to be captured could
be obtained as a co-benefit of existing and retrofitted
air quality control devices (AQCDs) that were designed to control other pollutants such as particulate,
sulfur oxides (SOx), and nitrogen oxides (NOx). For
some power plants, this is indeed the case, but more
often than not, additional products and technologies
will be necessary to boost the mercury capture cobenefits of these devices to reach the 1.2 lb Hg/TBtu
Nalco
(pounds of mercury per trillion British thermal units
fuel input) air emission limit specified in the MATS.
In seeking to control mercury air emissions, the control of mercury’s oxidation state is of critical importance. Upon combustion, all forms of mercury
present in coal will be converted to the elemental
form (Hg0), which readily evades capture by AQCDs
by virtue of its high volatility and insolubility in
water. Further downstream of the furnace, as the flue
gas cools, mercury can undergo reactions to become
oxidized to an ionic form (Hg2+). The extent of oxidation varies widely from plant to plant and depends
largely on the coal type and process conditions. In
this oxidized form, mercury is much more readily
captured by fly ash, sorbents, and scrubber liquors.
Thus, it is advantageous to maximize the conversion
of elemental mercury to the oxidized form in the
combustion gases to enhance capture.
Mercury oxidation is particularly advantageous in
situations in which a wet flue gas desulfurization
(WFGD) scrubber is in place for SOxcontrol, because
oxidized mercury is highly soluble in water and
readily dissolves in the scrubber liquor, often obviating the need for costly sorbent injection systems.
However, while pushing the mercury from the gas
phase into the scrubber liquor is a desirable outcome,
the mercury removal problem doesn’t end there.
Scrubbers necessarily produce wastewater streams,
and many states have stringent mercury limits on
wastewater effluent that must be met. Therefore,
products that can sequester and remove oxidized
mercury from solution in scrubber wastewaters are
often required. Further complicating matters is that
there are documented cases in which elemental mercury concentration in the flue gas actually increases
across the WFGD scrubber because of mercury re-
APC June 2012
emission, in which oxidized mercury in the scrubber
liquor undergoes chemical reduction back to the
volatile and insoluble elemental state (2-4). To comply with the new EPA MATS regulation, many plants
find it necessary to utilize WFGD scrubber additives
to improve the mercury capture efficiency by preventing mercury re-emission.
Nalco has developed technologies used to enhance
mercury oxidation, prevent mercury re-emission
from scrubbers, and bring scrubber wastewaters into
compliance with effluent limits. Brief overviews of
these technologies as well as two case studies that
document their use and effectiveness are discussed in
this article.
Mercury oxidation/speciation control
The capture of mercury in downstream particulate
control devices and scrubbers can be greatly enhanced
if the mercury is in the oxidized form (Hg2+) as opposed
to the elemental form (Hg0). To facilitate conversion to
the oxidized state, boiler additives can be applied to increase the relative proportion of oxidized mercury in
the flue gas. For example, Nalco has developed a solution product, MerControl® 7895 oxidant, which can be
applied directly to the coal before combustion or injected directly into the furnace. In numerous commercial trials, the fraction of mercury in the oxidized form
has been increased to as high as 90 percent (5). With the
use of such mercury oxidants, the performance of mercury sorbents, such as activated carbons, is significantly
enhanced, leading to a reduction in sorbent feed rates
and operating costs for the plant.
Mercury sequestration in WFGD liquors and
re-emission control
The application of mercury oxidation technology described above is especially well suited to plants that
have WFGD scrubbers installed for SOx control because Hg2+ is highly soluble in water and readily dissolves in the aqueous WFGD liquor. However, once
absorbed into the FGD liquor, ionic mercury can undergo chemical reduction to become insoluble and
volatile elemental mercury, which is re-emitted into
the scrubbed flue gas. To control this reaction, Nalco
developed a water-soluble polymeric product, MerControl® 8034, which can be added directly to the
FGD liquor (6). MerControl 8034 chemistry efficiently captures and precipitates ionic mercury out of
the liquor before it has the opportunity to undergo
chemical reduction and be re-emitted out the stack as
volatile elemental mercury. Furthermore it has been
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shown that the addition of MerControl 8034 chemistry to the WFGD scrubber has no impact on gypsum quality (3-6).
Mercury removal from WFGD wastewaters
To meet challenging limitations in the discharge of industrial wastewater heavy metals, companies often
turn to precipitation aids. To help meet mercury discharge limits, Nalco has developed a polymeric
chelant, Nalmet® 1689, with an exceptionally high
affinity for mercury. Upon binding mercury from solution, it forms large precipitates that readily settle
and filter to consistently attain extremely low mercury levels in the parts-per-trillion range in wastewater effluents.
Case studies
The technologies outlined previously can be applied
to reduce and control mercury emissions. They have
been applied in several commercial power plant operations, with excellent results. Two case studies follow,
each describing how these technologies have been applied and outlining how different control strategies
can be used to meet customers’ emissions limits.
Case study 1
Site description: 190 MWe (megawatt electrical)
pulverized coal-fired utility power plant firing highchlorine bituminous coal with selective catalytic reduction (SCR), cold-side electrostatic precipitator
(ESP), and WFGD.
The goal at this site was to reach greater than 85 percent mercury capture to meet mercury water discharge limits and an emissions limit of 0.008 lb
Hg/GWh.
Two separate issues needed to be addressed. First,
despite having an SCR, which has a co-benefit of oxidizing mercury in the flue gas, and a WFGD scrubber, which has a co-benefit of capturing oxidized
mercury, the plant was still unable to meet its emissions target. Therefore, to further enhance mercury
oxidation and capture, the mercury oxidant product
MerControl 7895 was applied to the coal feeders. Additionally, discharge limits for mercury in the plant’s
wastewater from the WFGD were in place. While the
primary goal of improved mercury capture from the
flue gas was attained through use of the oxidant, further containment and control of the mercury from the
WFGD wastewater was necessary. For this purpose,
Nalco’s polymer chelant product Nalmet 1689 was
added to the WFGD wastewater treatment system.
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June 2012 APC
Figure 1 shows the emissions of total and elemental
mercury at the stack plotted against a range of feed
rates of MerControl 7895 oxidant. Application at 265
mg product/kg of coal and above afforded greater
than 85 percent mercury capture and achieved the
target of 0.008 lb Hg/GWh. The net result of this
treatment was that the majority of the mercury was
captured in the WFGD liquor.
At this plant, the mercury concentration in the
WFGD wastewater has been in the range of 10,000 to
30,000 ppt. Moreover, with the treatment of MerControl 7895 chemistry causing the oxidized mercury in
the flue gas to increase, one would expect the amount
of mercury in the wastewater to increase substantially. Figure 2 shows that the treatment of the WFGD
wastewater with Nalmet 1689 chemistry lowered
Figure 1
Stack emissions of total and elemental
mercury as a function of MerControl 7895
oxidant feed rate
and maintained mercury concentrations in the clarifier effluent stream to well below 200 ppt, even as the
capture of the mercury from the flue gas in the
WFGD increased to more than 95 percent.
Figure 3, which plots the mercury content of the clarifier solids as a function of mercury capture from the
flue gas, shows that the Nalmet 1689 additive shifts
the captured mercury from the wastewater to the
clarifier solids. When the mercury is partitioned to
the clarifier solids, it is easily removed by standard
wastewater treatment equipment, and the solids can
be safely moved to a landfill.
The application of this customized, low-capital, twopronged program succeeded in pushing the mercury
from the gas phase to the water phase, and ultimately
to the solids that could be moved to a landfill, thereby
making the site compliant with both air and water
mercury emissions limits.
While the use of a mercury oxidant at this plant led to
more than 95 percent mercury capture from the flue
gas, cases have been documented in which the mercury removal efficiency of the WFGD was poor because of mercury re-emission, despite having mostly
oxidized mercury in the flue gas. Case study 2 illustrates one such case. This plant required an additive
to its WFGD liquor to remove the captured mercury
before it had the opportunity to undergo chemical reduction back to volatile elemental mercury and be reemitted out the stack.
Case study 2
Site description: Pulverized coal-fired utility power
plant firing high-sulfur bituminous coal with two
Figure 2
Figure 3
FGD wastewater mercury
concentration plotted versus percent
mercury capture from air
Total mercury content of the FGD
wastewater clarifier solids as a function of
mercury capture from air
APC June 2012
identical units generating about 500 MWe each. The
air pollution control devices on each unit include a
cold-side electrostatic precipitator (ESP) and a
WFGD scrubber. The WFGD scrubber utilizes limestone and forced oxidation for gypsum formation.
The goal of the demonstration was to use MerControl
8034 chemistry to capture all of the oxidized mercury
entering the WFGD and prevent mercury re-emission.
Site 1 was experiencing unexpectedly low mercury
capture across its WFGD scrubbers because of mercury re-emission. The flue gas coming into the
WFGD scrubbers was approximately 80 percent oxidized, but the total mercury capture across the
WFGD was much lower, around 20 to 40 percent This
indicated that the oxidized mercury was most likely
being chemically reduced in solution and re-emitted
out the stack. This case study represents a demonstration of the capability of the WFGD additive, MerControl 8034, at capturing the ionic mercury out of
the WFGD basin and thus preventing it from being
re-emitted out the stack. Measurements were taken
throughout the demonstration to examine the soluble mercury content within the WFGD liquor of Unit
1, in which MerControl 8034 chemistry was added,
and Unit 2, in which no product was added and
served as a control. Gas phase mercury measurements were taken at the inlet of the WFGD and the
stack outlet to measure the amount of total mercury
in the flue gas as well as the percent oxidation of the
mercury.
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Figure 4 shows the percentage of mercury in the flue
gas entering the WFGD that is oxidized. It is consistently between 60 and 80 percent oxidized. The percentage of mercury capture is also shown in Figure 4.
Equations 1 and 2 detail how the percentage of mercury capture and the percentage of mercury oxidation are calculated, where HgT is total mercury and
Hg0 is mercury in the elemental state:
(
)
T
T
Hg capture [%] = Hg inlet - Hg stack ×100
T
Hg inlet
(
)
T
0
Hg oxidation [%] = Hg inlet - Hg ×100
T
Hg
[Eq. 1]
[Eq. 2]
As Figure 4 shows, during baseline conditions before
MerControl 8034 chemistry is fed into the system,
significantly less mercury is being captured by the
WFGD than what is coming in oxidized. Theoretically, if the WFGD is working efficiently, 100 percent
of the oxidized mercury should be captured in the
WFGD liquor. This indicates that the oxidized mercury is being reduced back to its volatile elemental
form in the WFGD liquor and being re-emitted out
the stack (mercury re-emission). The percentage of
oxidized mercury in the flue gas being re-emitted
from the WFGD is shown in Figure 5 and can be calculated according to equation 3:
(
)
0
0
Hg re-emission [%] = Hg stack - Hg inlet ×100 [Eq. 3]
T
0
Hg inlet - Hg inlet
MerControl 8034 is a polymeric coagulant developed
for the sequestration of ionic mercury in high-solids
Figure 4
Figure 5
Percentage of oxidized mercury in the
incoming flue gas to the WFGD (in purple),
and the percentage of total mercury captured
in the WFGD (in blue). Both lines correspond
to the y-axis on the left. The solid black line
is the MerControl 8034 feed rate, which
corresponds to the y-axis on the right.
Percentage of mercury being re-emitted as
elemental mercury in the WFGD (in red), and
the percentage of total mercury captured in
the WFGD (in blue). Both lines correspond to
the y-axis on the left. The solid black line is
the MerControl 8034 feed rate, which
corresponds to the y-axis on the right.
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June 2012 APC
environments such as those found in WFGDs. Figure 4 shows that once the MerControl 8034 chemistry is applied, the amount of mercury captured
increases and becomes equivalent to the amount of
oxidized mercury entering the WFGD scrubber,
indicating that the product leads to the complete
capture of oxidized mercury from the flue gas. Furthermore, Figure 5 shows that the amount of mercury re-emitted in the WFGD becomes negligible
once the MerControl 8034 chemistry is fed into the
system.
The successful prevention of mercury re-emission
was also evident in the measurements of mercury
concentrations in the WFGD liquors. Figure 6 shows
the soluble mercury concentrations measured in the
basins of the WFGD scrubbers in Units 1 and 2. Unit
1 was treated with MerControl 8034 chemistry at the
feed rates shown in black. Unit 2 served as the control with no added product. Once the MerControl
8034 treatment began in Unit 1, the soluble mercury
concentration dropped rapidly until it reached a
steady state at nearly zero. The significant decrease
in soluble mercury concentration indicates that the
MerControl 8034 chemistry is reacting and precipitating the soluble mercury out of solution.
There is a strong relationship between soluble mercury concentration in the WFGD liquor of Unit 1
and the percentage of mercury re-emission, as
Figure 6
The relative soluble mercury concentrations
in the WFGD liquor from the basins of Units
1 (red squares) and 2 (green triangles)
correspond to the y-axis on the left. The
black solid line represents the relative
MerControl 8034 chemistry feed rate, which
corresponds to the y-axis on the right.
shown in Figure 7. When the MerControl 8034
chemistry reacts with the soluble mercury, it precipitates the mercury out of solution before it has a
chance to undergo reduction back to the volatile elemental form. Thus, the addition of MerControl 8034
chemistry effectively removes the soluble mercury
from the WFGD and stops mercury re-emission
from occurring, leading to the capture of all oxidized mercury in the flue gas.
Conclusion
Maximizing the oxidation of mercury in the flue gas
is critical to achieving the greatest possible capture
rates with WFGD scrubbers. Such a strategy will enable many utilities to meet the air emissions limits
specified in the recent EPA MATS regulation. However, this will often prove to be only part of a larger
equation that must also include the control of the
mercury from the wastewater discharge at the back
end of a power plant to meet ever-tightening discharge limits. Also, many sites will eventually find
that the mercury capture efficiency of their WFGD
scrubbers is compromised because of the occurrence
of mercury re-emission. A scrubber additive that can
sequester the mercury within the WFGD basin itself
will probably be necessary in such cases.
Mercury emission rates are always highly coal and
site specific, and an intimate understanding of the
factors that affect mercury transformations and partitioning, as well as the ability to accurately measure
mercury in various process streams, will be essential
to developing a customized control strategy for any
power plant.
Figure 7
The relationship between percentage of
mercury re-emission and the soluble mercury
concentration in Unit 1’s WFGD basin
APC June 2012
Notes
MerControl, Nalmet, Nalco, and the logo are trademarks of
Nalco Company.
Nalco is an Ecolab company.
Ecolab is a trademark of Ecolab USA Inc.
References
1. United States Environmental Protection Agency, Final
Mercury and Air Toxic Standards (MATS) for Power Plants,
www.epa.gov/mats/actions.html.
2. Miller, CE, Feeley, TJ, Aljoe, WW, Lani, BW, Schroeder, KT,
Kairies, C, McNemar, AT, Jones, AP, and Murphy, JT.
“Mercury Capture and Fate Using Wet FGD at Coal-Fired
Power Plants,” in DOE/NETL Mercury and Wet FGD R&D,
August 2006. www.netl.doe.gov/index.html.
3. Blythe, G, Currie, J, and DeBerry, D. Bench-scale Kinetics
Study of Mercury Reactions in FGD Liquors, Final Report:
DE-FC26-04NT42314 for the National Energy Technology
Laboratory, Austin, TX, 2008.
4. Munthe, J, Xiao, Z, and Lindqvist, O. Water, Air, & Soil
Pollution, 1991 (56) 621-630.
5. Keiser, BA, Meier, J, Shah, J, and Lu, J. Paper presented at
MEGA Symposium, Baltimore, MD, 2010.
6. Stiles, RL, Zinn, PJ, Lu, JV, Michels, JJ, Leigh, AM, and Keiser,
BA. Preprints of Symposium–Am. Chem. Soc., Division of
Fuel Chemistry: San Francisco, CA, 2010 (55) 164-166.
APC
Richard Mimna ([email protected]) is a senior research chemist with Nalco (www.nalco.com). He holds a
PhD in chemistry from the EPFL in Lausanne, Switzerland. Richard has been working in the area of mercury control in the utility industry for the past four years.
Rebecca Stiles is a senior research chemist at Nalco ([email protected]) in the Air Protection Technologies
group. Rebecca earned her BS degree in chemistry from
Union College in Schenectady, NY, in 2003 and her PhD
in chemistry from the University of North Carolina,
Chapel Hill, NC, in 2007. Rebecca’s research at Nalco focuses on reducing emissions from coal-burning power
plants, and specifically on mercury and carbon dioxide.
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Jerry Yuan is a senior research scientist at Nalco (jyuan1
@nalco.com) in the Air Protection Technologies group.
Jerry earned his bachelor of engineering, master of engineering, and PhD degrees in thermal power engineering
(mechanical/chemical) from the Huazhong University
of Science and Technology in China. Jerry has worked on
a variety of projects for Nalco, including modeling the
company’s PARETO mixing technology for improved
operational efficiencies, and working on lowering carbon dioxide and mercury emissions from coal-burning
power plants.
Bruce Keiser is a research fellow at Nalco (bkeiser
@nalco .com), with the responsibility for research and development in Air Protection Technology and nanotechnology as it pertains to the generation of power, reaching
new oil, and water use in industrial processes. Bruce received a BS degree in chemistry from Grove City College
in Grove City, PA, and his PhD in inorganic chemistry
from the University of Wyoming. Dr. Keiser has been instrumental in the growth and deployment of mercury
control technologies for the Air Protection Technology
Group. Dr. Keiser is a published author with more than
20 technical articles in peer-reviewed journals, a recognized expert in colloidal silica, nanotechnology, and mercury control strategies for air and water as it applies to
coal-fired power plants. He is an inventor with more than
20 granted patents in the United States and abroad, with
another nine published U.S. patent applications.
John Meier is the global mercury product line manager
for the Air Protection Technologies Division at Nalco
([email protected]). John is responsible for all Nalco
mercury air emissions control technologies, new product research and development, customer trials and
analysis, and customer accounts. John graduated in
2004 from the University of Wisconsin-Milwaukee with
a BS degree in civil engineering. John has more than 7
years of work experience in mercury air emissions control. Before his current role, he served as a mercury technology specialist and an R&D project manager at Nalco
Mobotec, and as a research assistant at the University of
Wisconsin.